The present invention relates to improvements in precision strain-gage transducers which may be readily compensated for temperature effects upon zero-balancing, and, in one particular aspect, to high-performance transducers, such as miniature load beams, wherein temperature-induced instability of zero balance of strain-gage networks is uniquely and advantageously counteracted by way of resistance units readily abradable to adjust internal network resistances at room temperature and effect compensation in accordance with results of predetermined measurements at different temperatures and of related computations based upon both the measurement data and a factor associated with the resistance material.
Accurate measurements characterizing such phenomena as force, torque, weight and pressure are often performed through the instrumentality of so-called strain-gage transducers, wherein electrical-resistance strain gages respond to elastic deformations of sensing elements undergoing loading. Such transducers have become well known in a variety of sizes and forms, and they can be expected to yield measurements with most remarkable exactness even under extremely severe operating conditions when constructed and compensated with great care. Among the numerous causes of possible error which can afflict precision transducers, some of the most vexing are associated with adverse effects of temperature changes. Virtually every portion of a transducer sensing element and its gages and its wiring systems can respond to thermal variations in some way tending to degrade resultant measurements, and it has therefore become common practice for manufacturers of such assemblies to incorporate temperature-compensation provisions into them. The compensation techniques and practices may vary, depending upon the nature and extent of thermal problems encountered in a particular situation, as well as the time and expense which can be justified in achieving desired degrees of improvement.
When properly excited and coupled to impress its output upon a display or control stage, a strain-gage transducer in the unloaded condition might always be expected to signal a related zero output, and simple adjustable balancing resistances in adjacent arms of its strain-gage bridge network may in fact suffice transiently to trim the network to that zero-output condition when the temperature remains fixed. However, temperature excursions no greater than those likely to be encountered in many applications will most often have the highly undesirable effect of upsetting the pre-adjusted zero-balance, and consequently the accuracy of any measurements made without accounting for the imbalance. Repeated temperature re-cycling of the transducer, along with repeated bridge re-balancings, may well add significant manufacturing costs and difficulties without altogether eliminating the possibility of further unbalancing due to subsequent temperature changes, and, further, one cannot expect that zero-balance resistors built into transducers will be accessible for periodic adjustments by the user because such devices are commonly both remotely located and permanently hermetically sealed. It has been known previously to calculate the amount of temperature-compensating resistance which should be introduced in an arm of a transducer bridge circuit to achieve a desired balance, and to select and solder into place in an appropriate arm a small "charted" or predetermined length of temperature-sensitive wire which would provide that resistance, but exact lengths were often difficult to maintain effectively, due to such factors as the shunting effects of solder, and the attendant manual labor and skills involved were negative factors also.
In accordance with teachings and related practices of the present invention, zero-balance resistances which are in some respects counterparts of prior balance resistances are also utilized, but permanent temperature compensation for zero balance purposes is brought about with the aid of further adjustable resistances which have known temperature dependencies and which are included in both of two adjacent arms of a bridge circuit as exposed foil elements accessible for critical adjustment by abrasion. By way of calculations based in part upon measurements obtained while the unloaded transducer is maintained at different temperatures, and by way of control measurements performed automatically or by an operator while one of the temperature-sensitive foil elements undergoes abrasion, the unsealed strain-gage transducer may be accurately temperature-compensated for a zero-balancing which will hold for subsequent operations of the finished product within a useful range of temperatures.
Among prior U.S. patents which deal with various aspects of thermal compensation in respect of strain gage transducers are U.S. Pat. Nos. 2,801,388 and 3,178,938.